In recent years the need for global data networking capability has rapidly expanded. In order to meet this need, broadband satellite communication systems have been proposed as an alternative to land-based communication systems. One type of satellite data communication system is described in a number of U.S. patents assigned to Teledesic LLC of Kirkland, Washington, the assignee of this patent application. These patents include U.S. Pat. Nos. 5,386,953; 5,408,237; 5,527,001, 5,548,294; 5,641,135; 5,642,122, and 5,650,788 and other pending applications which describe a satellite communication system that employs a constellation of low-Earth orbit (LEO) satellites and an Earth-fixed cell transmission scheme. Data to be sent from one location on the Earth to another location is transmitted from a ground terminal located within a cell to a satellite that is serving the cell via an uplink data channel. The data is routed through the constellation of LEO satellites to a satellite that is servicing the cell within which the designated ground terminal is located. The latter satellite transmits the data to the ground terminal via a downlink data channel. Thus, the constellation of LEO satellites and the ground terminals form a data communication network wherein each ground terminal and satellite forms a node of the network.
For a LEO satellite data communication system to be competitive with conventional ground-based data communication systems, it must support broadband applications at a relatively low cost. In order to allow the transmission of real time video, full duplex telecommunications, and other bandwidth intensive applications, it is estimated that the data transmission rate for a LEO satellite data communication system must approach 400 Megasymbols/second. This data rate is an order of magnitude above any data rate currently used in satellite communication systems.
Designing an inexpensive receiver for use as a ground terminal or satellite that can receive information at such a high data rate has proven to be problematic. The traditional approach to creating a high data rate receiver has been to buffer the received data and process the data after reception (i. e., not in real-time). An example of such a buffered receiver is shown in FIG. 1. Data received from a satellite on the downlink or from a ground terminal on the uplink is applied to an analog to digital converter 10, where it is converted into a series of digital samples. Because the samples arrive at a rate that is faster than the rate at which they can be processed, the samples are stored in a buffer 12 and decoded by a demodulator 14 at the rate at which the demodulator is able to operate. Processing the data after reception is not a satisfactory solution in a LEO satellite data communication system for several reasons. Most importantly, the size of the buffers necessary to store the received data would greatly increase the cost of the ground terminals and satellites. Buffering also places strict limits on scheduling of data transmissions between nodes in the network to ensure that the buffers at a particular node do not overflow. The additional overhead necessary to avoid buffer overflow adds to the complexity of managing the communication system. Moreover, the overall throughput of the satellite communication system may be diminished due to a bottleneck at any one of the satellites or ground terminals.
An alternative approach to creating a high data rate receiver is to distribute the data processing tasks of a receiver among multiple channels that are connected in parallel. By processing the data in parallel channel, the overall data processing rate of any particular channel is sufficiently reduced to allow the data to be processed in realtime, i.e., as the data is received. A parallel digital modem architecture for use in deep space communications has been suggested in a Jet Propulsion Laboratory report entitled "Parallel Digital Modem Using Multirate Digital Filter Banks," published Aug. 15, 1994, and co-authored by one of the inventors of the present application. This article is herein incorporated by reference.
Although such parallel processing works well when a continuous data stream is received by a receiver, it has not been possible to implement a parallel receiver in a LEO satellite data communication system due to the manner in which data is transmitted between the satellites and the ground terminals. Traditional deep space satellite communication systems transmit data more-or-less continuously, allowing sufficient time for a ground terminal receiver to synchronize with the incoming data transmission. In contrast, the proposed LEO systems have adopted discontinuous interlink and downlink transmission schemes. That is, data is transmitted between satellites or from a servicing satellite to a ground terminal in a burst that may start at any time and be of variable duration. Because a receiver does not know when a data burst will be received or the length of the burst, prior art parallel receivers have been unable to quickly synchronize with the received data stream. The inability to quickly synchronize with the received data stream has prevented parallel transmitters and receivers from being considered as a viable alternative for high data rate transmitters and receivers in satellite data communication systems.
Given the shortcomings in the prior art, there is a need for a relatively low-cost data transmitter and receiver that can accommodate high transmission rates of bursty data having random durations.